(1) Field of the Invention
The present invention relates to a thermoplastic resin composition in which excellent mechanical toughness, durability and solvent resistance of polyamide resins are still kept up and in which hygroscopicity, moldability and impact resistance are improved, and it relates to a method for preparing the resin composition. This composition of the present case can be widely utilized as materials for electrical and electronic machine parts, automobile parts and the like.
(2) Description of the Prior Art
In recent years, polyamide resins are getting important more and more in the industrial field. With regard to the polyamide resins, thermal stability is extremely excellent, hardness is very high, and heat resistance is also good, but impact resistance is low and hygroscopicity is high. Therefore, applications of the polyamide resins are now limited inconveniently.
As technical means for improving the impact resistance of the polyamide resins, it is already known to use a reactive group-containing polyolefin elastomer such as a maleic anhydride-modified polyolefin elastomer or an epoxy group-containing polyolefin elastomer as an impact resistance improver.
For the purpose of improving the hygroscopicity of the polyamide resins, there have been also suggested a method of melting and mixing polystyrene or styrene-acrylonitrile copolymer (Japanese Patent Publication No. 40-7380), a method of melting and mixing an .alpha.,.beta.-unsaturated dicarboxylic anhydride or an epoxy group-containing polystyrene (Japanese Patent Unexamined Publication No. 60-86162), a method of mixing a glass fiber and a thermoplastic resin (Japanese Patent Publication No. 48-13944, and GB Patent Nos. 1,241,361 and 1,552,352), and other methods.
However, the above reactive group-containing polyolefin elastomer as the impact resistance improver is a substantially rubbery polymer for its use purpose and has a glass transition temperature of room temperature or less. Therefore, in order to heighten the impact resistance, it is conceived to increase the amount of the reactive group-containing polyolefin elastomer, but if the content of the latter is merely increased, important characteristics of the polyamide resins, for example, tensile strength, surface hardness and thermal deformation resistance, i.e., thermal dimensional stability deteriorate disadvantageously.
With regard to the method of melting and mixing polystyrene or styrene-acrylonitrile copolymer in order to improve the hygroscopicity of the polyamide resins, articles produced thereby have a tendency to peel off in the form of layers because of the poor compatibility of vinyl copolymer to the polyamide resin, so that mechanical strength degrades noticeably. For this reason, it is known that this method cannot provide good molding materials.
As for the method of mixing a glass fiber and a thermoplastic resin with a polyamide resin, the hygroscopicity is effectively lowered, but molding devices such as the screw of an extruder, a mold and the like are liable to wear away. In addition, secondary treatments such as ion plating, sputtering, coating and the like are difficult, and fluidity is also poor inconveniently.
In the method of melting and mixing an .alpha.,.beta.-unsaturated dicarboxylic anhydride or an epoxy group-containing polystyrene, its compatibility with the polyamide resin is improved by the reaction between its reactive group and the residual reactive group of the polyamide resin, and in consequence, the improvement in hygroscopicity is also confirmed. However, this method has the drawback that the impact resistance deteriorates.
The inventors of the present application have intensively researched to solve the conventional disadvantages, i.e., to improve impact resistance and hygroscopicity/while simultaneously maintaining mechanical and thermal characteristics of the polyamide resins. As a result, it has been found that a thermoplastic resin composition obtained by blending a specific multi-phase structure thermoplastic resin with the polyamide resin improves the impact resistance and hygroscopicity simultaneously, keeping up the mechanical and thermal properties, and that in manufacturing the above-mentioned composition, the above melting and kneading operations should be at a specific optimum temperature. On the basis of these findings, the present invention has been completed.